The third step in unlocking the Lakna Rokee shrine is actually pretty straightforward — you just have to complete the By Firefly’s Light side quest. You’ll pick up this quest by visiting Lasli in her house at night (that sounded less creepy in our head). She’ll tell you how much she misses catching fireflies.
Your job is to bring the fireflies to her — five of them, to be precise. If you’re lucky (or a hoarder like we are), you might already have the fireflies in your inventory. If you don’t, head back outside and crouch. There are usually a few outside her house at night. (You can also occasionally buy them from Beedle.)
Once you have (at least) five of them, choose the fireflies out of your inventory, hold five of them, then drop them inside her house. She’ll reward you with another 50 rupees. (And then, before you leave, jump around her room and reclaim all of those fireflies because they’re a pain to catch out in the wild.)